This invention relates to a shift control apparatus used in an automatic transmission for a vehicle having at least one frictional engaging element and a electromagnetic valve to control the hydraulic pressure supplied to the frictional engaging element, achieving a predetermined speed ratio by engaging the frictional engaging element, in which speed-shift shocks when an engine is started after a longtime standstill condition and a first shift is performed are reduced.
The automatic transmission for a vehicle selectively supplies hydraulic fluid to frictional engaging elements such as clutches and brakes to connect a desired rotary element in its gear system to an input shaft of the transmission or fix the element to a transmission casing, thereby automatically changing the speed ratio according to operation conditions of the vehicle.
Such an automatic transmission for a vehicle is required to be small in speed-shift shocks to protect various parts and components and maintain comfortable drive feeling. For this purpose, an automatic transmission for a vehicle has been proposed which uses a proper electronic control over the hydraulic pressure and its supply timing to frictional engaging elements, aiming for reduced speed-shift shocks.
An example of the structure of such an automatic transmission for a vehicle is shown in FIG. 10.
Referring to FIG. 10, a crank shaft 12 of an engine 11 is integrally connected with an impeller 14 of a torque converter 13. The torque converter 13 has the impeller 14, a turbine 15, a stator 16, and a one-way clutch 17. The stator 16 is connected to a transmission casing 18 through the one-way clutch 17. By the function of the one-way clutch 17, the stator is allowed to rotate in the same direction as the crank shaft 12 but is not allowed to rotate in the reverse direction. The torque transmitted to the turbine 15 is transmitted to an input shaft 19 of a gear transmission apparatus (hereinafter referred to as the "transmission input shaft") to achieve four forward speeds and a single reverse speed disposed at the rear of the torque converter 13.
The gear transmission apparatus comprises three clutches 20, 21, and 22, two brakes 23 and 24, one one-way clutch 25, and one ravigneaux type planetary gear mechanism 26. The ravigneaux type planetary gear mechanism 26 comprises a ring gear 27, a long pinion gear 28, a short pinion gear 29, a front sun gear 30, a rear sun gear 31, and a carrier 32. The carrier 32 rotatably supports the pinion gears 28 and 29 and is rotatably engaged with the transmission input shaft 19.
The ring gear 27 is connected to a transmission output shaft 33. The front sun gear 30 is connected to the transmission input shaft 19 through a kickdown drum 34 and a front clutch 20. Furthermore, the rear sun gear 31 is connected to the transmission input shaft 19 through a rear clutch 21. The carrier 32 is connected to a transmission casing 18 through a low reverse brake 24 and the one-way clutch 25 and to the transmission input shaft 19 through a 4th-speed clutch 22 disposed at the rear end of the gear transmission apparatus. The kickdown drum 34 is integrally connectable to the transmission casing 18 by a kickdown brake 23. Torque passed through the ravigneaux type planetary gear mechanism 26 is transmitted from a drive gear 35 mounted to the transmission output shaft 33 to the drive shaft side of driving wheels (not shown).
The clutches 20 to 22 and the brakes 23 and 24 as frictional engaging elements individually comprise hydraulic mechanisms provided with engaging piston devices or servo mechanisms. These hydraulic mechanisms are operated through a hydraulic control unit (not shown) by hydraulic fluid generated by an oil pump 36 connected to the impeller 14 of the torque converter 13.
Detailed structure and functions of the mechanisms are already known, for example, in Japanese Patent Publication Laid-open 58-46248/1983, 58-54270/1983, or 61-31749/1986. Thus, selective engagement of various frictional engaging elements is achieved according to the position of a shift lever provided in the driver's seat of the vehicle (not shown) selected by the driver and operation conditions of the vehicle, and various speed ratios are automatically achieved through the hydraulic control unit according to instructions from an electronic control unit to control the operation conditions of the engine 11.
The select pattern of the shift lever includes P (parking), R (reverse), N (neutral), D (automatic three forward speeds or automatic four forward speeds), 2 (automatic two forward speeds), and L (fixed to the 1st speed) positions. With the shift lever set to the D position, when an auxiliary switch (over-drive switch, not shown) is operated, the automatic three forward speeds or the automatic four forward speeds can be selected. The functions of the individual functional engaging elements when the shift lever is set to the individual positions are shown in FIG. 11. In the figure, symbol ".largecircle." indicates that an engagement condition is by hydraulic operation, and symbol " " indicates that the engaging is achieved only when the L position is selected.
With the vehicle in standstill, when the shift lever is operated from the N position to a running position, that is, to the D position or the R position, an engaging hydraulic pressure P.sub.0 to a frictional engaging element to achieve the speed ratio is replaced with a target duty ratio d.sub.0 of a hydraulic pressure control valve incorporated in the hydraulic control unit to adjust the engaging hydraulic pressure P.sub.0 and controlled. In this case, the target duty ratio d.sub.0 is set according to a basic duty ratio d.sub.B read from a three-dimensional graph as shown in FIG. 3 using an oil temperature T.sub.0 of automatic transmission oil and an engine speed N.sub.E, for example, as variables. Thus, a predetermined engaging hydraulic pressure P.sub.0 as shown in FIG. 4 corresponding to the target duty ratio d.sub.0 which is set according to the basic duty ratio d.sub.B is supplied to the frictional engaging element.
FIG. 4 shows an example where a hydraulic pressure control valve of a type which closes when unenergized to control the engaging hydraulic pressure P.sub.0 is used. On the other hand, when a hydraulic pressure control valve of a type which opens when unenergized, the engaging hydraulic pressure tends to decrease as the target duty ratio d.sub.0 increases.
In a prior art automatic transmission for a vehicle shown in FIG. 10 and FIG. 11, in which the target duty ratio d.sub.0 of the hydraulic pressure control valve incorporated in the hydraulic control unit is electronically controlled to adjust the engaging hydraulic pressures P.sub.0 to a plurality of frictional engaging elements, and these frictional engaging elements are selectively engaged to achieve a plurality of speed ratios, when the engine 11 is in a standstill condition over a long period of time, automatic transmission oil held in the oil path of the hydraulic control unit tends to fall down to an oil reservoir. As a result, when a running position is selected to start the vehicle immediately after the engine is started, automatic transmission oil does not spread over the entire oil path of a hydraulic pressure control circuit, and a delay in engagement tends to occur in the engaging side frictional engagement element with the engaging hydraulic pressure P.sub.0 according to the normal basic duty ratio d.sub.B shown in FIG. 3, resulting in speed-shift shocks.
Changes in the target duty ratio d.sub.0 of the hydraulic pressure control valve and a rotation speed N.sub.T of the turbine 15 of the torque converter 13 are shown in FIG. 5.
As shown in FIG. 5, at the beginning of a shift, the target duty ratio d.sub.0 of the hydraulic pressure control valve indicated by the solid line rises so that engagement of the engaging side frictional engaging element is slowly achieved. Then, the target duty ratio d.sub.0 is reverted back to 0% and a shift end signal is transmitted. In this case, in a normal condition where automatic transmission oil is held in the oil path of the hydraulic control unit, the rotation speed N.sub.T of the turbine 15 of the torque converter 13 becomes zero during this time. Then, shift operation by the engagement of the engaging side frictional engaging element is completed, and speed-shift shocks almost do not occur.
However, after the engine is in a standstill condition for a long period of time, when a running position is selected to start the vehicle immediately after the engine is started, since automatic transmission oil held in the oil path of the hydraulic control unit has fallen down to the oil reservoir, a delay in supplying hydraulic oil to the engaging side frictional engaging element tends to occur when the running position is selected to start the vehicle immediately after the engine 11 is started. As a result, the rotation speed N.sub.T of the turbine 15 of the torque converter 13 is not zero when the shift end signal is transmitted. Changes in the rotation speed N.sub.T of the turbine 15 of the torque converter 13 are shown by the solid line in the figure. Thus, after a while from the transmission of the shift end signal, the rotation speed N.sub.T of the turbine 15 of the torque converter 13 becomes zero, and the shift operation is substantially completed. As a result, speed-shift shocks occur in association with the delay in engagement of the engaging side frictional engaging element.
Such a problem tends to conspicuously occur in a vehicle equipped with a hydraulic control unit of long oil path, especially, a vehicle equipped with an automatic transmission for a large vehicle corresponding to a large-displacement engine.